1. Field of the Invention
This invention relates to curing of polyepoxide resins to produce hard, insoluble, infusible films, castings and adhesives.
2. Description of the Prior Art
Polyglycidyl ethers, particularly those prepared from a dihydric phenol such as Bisphenol A, i.e., 2,2-bis(4-hydroxyphenyl) propane, and an epihalohydrin such as epichlorohydrin, also referred to as epoxy resins, epoxide resins, polyepoxide resins or polyepoxides, have become increasingly important commercially in recent years. When cured, these thermosetting resins form insoluble, infusible films, pottings, castings, adhesives, and the like, and are markedly superior in their physical, chemical, and electrical properties to many other cured thermosetting resins. They exhibit low shrinkage during curing. The combination of hardness and toughness exhibited by the cured resins, their high adhesive strength, resistance to degradation by solvents and other chemicals and their electrical properties, such as dielectric constant and resistivity, are outstanding. At the same time, these properties can be varied within wide limits depending on the end use intended for the resin. Of the wide variety of hardeners, curing agents, or homopolymerization catalysts which have been used to cure polyepoxide resins, no one is suitable for all applications, and many have serious drawbacks no matter what the application.
Carboxylic acids and anhydrides are used as curing agents. However, polyepoxide compositions containing them must be cured at elevated temperatures for some time.
Polyphenols are also employed as curing agents for polyepoxide resins, albeit, somewhat less extensively, as they require relatively high temperatures for effective cures.
Catalysts, both Lewis acids and Lewis bases may be used to cure epoxy resins by homopolymerization. Lewis acids used commercially are typically complexes of boron trifluoride and the like. Types of complexes that cure at room temperature, for example, boron trifluoride/glycols, while rapid, are poisoned by atmospheric or surface water, alkaline fillers and alkaline substrates, for example, Portland Cement construction on which they are placed. Lewis bases, typically tertiary amines and tertiary amine salts, onium bases, tertiary phosphines and certain organic sulfides require an elevated temperature to complete cure.
Primary and secondary polyamines of various types are frequently employed as curing agents for polyepoxide resins, but the results obtained vary depending on a number of factors, such as the particular amine, the polyepoxide resin employed, the curing temperature, and so on.
Many amines fail to give satisfactory results over a wide range of curing conditions. Unmodified cycloaliphatic and aromatic polyamines require a high temperature, for example, 125.degree.-200.degree. C., to bring about full cure. These same types when modified with certain materials such as plasticizers and flexibilizers and acidic accelerators will cure at room temperature, but the cure rate becomes extremely slow or non-existent at temperatures less than 5.degree. C. and especially less than 0.degree. C.
The most widely used types of curing agents for epoxy resins at room temperature are aliphatic primary and secondary polyamines used as such or modified, for example, to produce the so called amidoamines and reactive polyamide resins. Curing times necessary when using certain aliphatic amines are often longer than desirable or practical, while with other more reactive accelerated aliphatic polyamines the formulated composition has limited usable pot life even at room temperature. The amine/epoxy cure rate is temperature dependent. Most aliphatic amines will cure epoxy resins at room temperature (25.degree. C.), and some even as low as 5.degree. C. The cure rate at lower temperatures; however, is often too long to be practical. Below 5.degree. C. the cure rate of activated aliphatic polyamines is decreased to the point of unworkability.
Polymercaptans are employed to cure polyepoxides. They are less dependent upon temperature and mass for their cure rates. Most polymercaptan hardener systems are designed to produce rapid thin film cures even at low temperatures (less than 0.degree. C.). Useful cures can be achieved even as low as -18.degree. C. It is further desirable to have the thin film set time and the large mass usable life (pot life) as close as possible. This indicates relative non-dependence on exothermic and ambient temperature for cure, and provides a more consistent cure time which is less dependent on ambient conditions. However, to obtain fast cure rates with polyepoxides, polymercaptans must be compounded with a catalyst. Tertiary amines such as 2,4,6-tri (dimethylaminomethyl) phenol, benzyl dimethylamine, and dimethylaminomethyl phenol are commonly used.
A number of tertiary amines have been described as catalysts for the mercaptan epoxy reaction. These include dimethylaminomethyl phenol. See U.S. Pat. No. 2,789,958--Fettes et al--Apr. 23, 1957, Examples 2,3,4,5 and 5A. 2,4,6-tri (dimethylaminomethyl) phenol is shown in U.S. Pat. No. 3,090,793--Casement et al--May 21, 1963. See Examples 2 through 5, 7 through 10, 12 and 13. Example 11 describes a tertiary phosphine in combination with the tertiary amine.
Use of 2,4,6-tri (dimethylaminomethyl) phenol or benzyl dimethylamine is shown in U.S. Pat. No. 3,297,635--Bergman et al--Jan. 10, 1967, (all examples). 2,4,6-tri (dimethylaminomethyl) phenol, dimethylaminomethyl phenol, dicyandiamide and pyridine are shown in all examples of U.S. Pat. No. 3,310,527--DeAcetis et al--Mar. 21, 1967, except Examples 4,6 and 7 where thioethers are catalysts, e.g., dibutyl sulfide and dioctyl sulfide. Example 6 also describes a tertiary phosphine, triphenyl phosphine, and a quaternary ammonium compound, benzyl trimethylammonium chloride, as catalysts.
Benzyl dimethylamine, dimethylaminomethyl phenol, triethanolamine and 2,4,6-tri (dimethylaminomethyl) phenol are shown as catalysts in Examples 2 through 7,9,11 and 12 in U.S. Pat. No. 3,355,512--DeAcetis et al--Nov. 28, 1967. Benzyl dimethylamine is shown in all examples in U.S. Pat. No. 3,369,040--DeAcetis et al--Feb. 13, 1968.
Benzyl dimethylamine (Examples 1 through 9), certain tertiary phosphines including triphenyl phosphine, tricyclohexyl phosphine and triamyl phosphine (Example 9) and certain quaternary ammonium compounds including benzyl trimethylammonium chloride, phenyl tributylammonium chloride and benzyl trimethylammonium borate (Example 11) are shown in U.S. Pat. No. 3,411,940--Lopez et al--Nov. 19, 1968. In U.S. Pat. No. 3,448,112--DeAcetis et al--June 3, 1969, is disclosed 2,4,6-tri (dimethylaminoemethyl) phenol (Examples 1 and 2), dimethylaminomethyl phenol (Example 3) and a thioether, dibutyl sulfide (Examples 4 and 7). Also pyridine and a thioether, dioctyl sulfide, a tertiary phosphine, triphenyl phosphine and a quaternary ammonium compound, benzyl trimethylammonium chloride, are shown in Example 6. Triethylenediamine is shown in Examples 9 and 10 and N,N,N',N'-tetramethylbutylenediamine is shown in Example 12 of U.S. Pat. No. 3,472,913--Ephraim et al--Oct. 14, 1969. U.S. Pat. No. 3,505,166--Jones et al--Apr. 7, 1970, shows the use of 2,4,6-tri (dimethylaminomethyl) phenol in Example 2.
Regarding present commercial practices using tertiary amines, 2,4,6-tri (dimethylaminomethyl) phenol is the most widely employed catalyst, while N,N,N',N'-tetramethylbutylenediamine, dimethylaminomethyl phenol and benzyl dimethylamine are also used.
The above patents claim for the most part novel polymercaptans as curing agents for epoxy resins and exemplify their utility using the well-known tertiary amine catalysts.
U.S. Pat. No. 3,291,776--Newey et al--Dec. 13, 1966, describes an unusual class of catalyst for the epoxy/mercaptan reaction, namely, the class of organic sulfides of the formula R--S--R where R is an aliphatic , alicyclic or aromatic radical having no more than 25 carbon atoms such as 2,2-thiodiethanol, dibutyl sulfide, 3,3-thiodipropanol and N-propylphenol sulfide. This class of catalyst has not achieved significant commercial acceptance due to the relatively slow catalytic action compared to tertiary amines.
U.S. Pat. No. 3,363,026--Schroll--Jan. 9, 1968, describes another unusual class of catalysts for the epoxy/mercaptan reaction comprising bicyclic, fused ring amines containing only carbon, hydrogen and nitrogen and having a nitrogen atom in at least one of the bridgehead positions, the nitrogen being connected to three different saturated carbon atoms and bearing an unshared pair of electrons, such as triethylenediamine and quinuclidine. However, this class exemplified by the preferred triethylenediamine possesses certain disadvantages as mercaptan/epoxy catalysts. It is a crystalline solid having a tendency to sublime even at room temperature and absorb moisture. Hence, its incorporation into a mercaptan hardener must be done under controlled conditions and temperatures. Further, containers of this mixture must be well sealed to prevent loss of reactivity due to catalyst volatilization and to prevent excessive absorption of moisture from the atmosphere. Further, while catalyzing the mercaptan/epoxy reaction, triethylenediamine appears to lose effectiveness. That is, while this catalyst as well as other fused ring catalysts promote a very rapid initial reaction, soon after and well before all epoxy and mercaptan groups are consumed, the reaction rate drops dramatically. Thus, the amine becomes bound up in the forming epoxy/mercaptan matrix through some form of complex formation characteristic of this class of tertiary amines. This deactivation effect can be demonstrated on a thin casting of an epoxy system containing a mercaptan accelerated with catalyst by measuring Barcol hardness as a function of time. When doing this, a plateau (hardness development stagnation) is reached quite early in the reaction process. Hardness development is taken as a representation of the degree of cure. Hardness then increases at a much slower rate to the final ultimate hardness or fully cured state.
Japanese Provisional Patent Publication (Japan Kokai) No. 17299/1976 [26 (5) K211] Feb. 12, 1976 to Miura, describes use of diazabicycloalkenes such as DBU (1,8-diazabicyclo (5,4,0) undecene-7) and its salts for reaction with epoxy resins. While this group of new catalysts is less volatile than the Schroll group, for example, triethylenediamine, it suffers from the disadvantage of reacting with moisture or water even at room temperature to form new compounds (primary amines) which present practical compounding and storage difficulties. Also, it exhibits significant toxic health hazards to humans.
With regard to the tertiary phosphines used by Casement et al and DeAcetis et al to catalyze the reaction of their novel mercaptans with polyepoxides, it is noted that this class of catalysts has not achieved commercial significance due to its relatively slow cure rate compared to the corresponding tertiary amines. In many cases members of this class also possess significantly higher toxicity relative to the tertiary amines.
"Onium" bases defined as highly ionized compounds containing ammonium, phosphonium, arsonium, stibonium, sulfonium, iodonium and thallonium cations may be divided into two types for the purpose of catalyzing the epoxy/mercaptan reaction; (1) those that are extremely slow in catalyzing the reaction, typically bases with inorganic anions excepting hydroxyl such as benzyl trimethylammonium chloride, phenyl tributylammonium chloride, benzyl trimethylammonium sulfate and benzyl trimethylammonium borate exemplified by DeAcetis et al and Lopez et al referred to above, the slow rate probably due to lack of solubility in in the epoxy/mercaptan media which does not normally contain water; and (2) those that are extremely fast in catalyzing the reaction; typically oil soluble onium bases such as the quaternary ammonium commpounds with hydroxyl or organic substituted anions such as the tetra alkyl ammonium hydroxides, alkoxides and phenoxides; benzyl trialkyl ammonium hydroxides, alkoxides and phenoxides and materials such as choline base and the like. All such "onium" bases that provide rapid catalysis of the epoxy mercaptan reaction are believed to do so by the general base catalysis mechanism described by Tanaka and Mika in "Epoxy Resins" edited by May and Tanaka and published by Marcel Dekker, 1973. See page 164 reactions 26 and 27. These catalysts apparently generate the active ionic species (the mercaptide ion) when the "onium" base and mercaptan are mixed and stored in the absence of epoxy resin, thus bringing about rapid gelation of the mercaptan on storage. Such gels have little value and three component systems whereby the epoxy resin, mercaptan and catalyst must be stored separately present extreme handling difficulties for the user. Active "onium" bases as catalysts for the epoxy/mercaptan reaction have not achieved commercial acceptance.